The present invention relates to a proces for the suspension smelting of ferriferous sulfide concentrates.
The horizontal, free-injection suspension processes according to U.S. Pat. Nos. 578,912 and 1,164,653 operating with cold and preheated air can be regarded as the basic methods for suspension roasting and smelting of sulfide concentrates. The fitting of a vertical suspension oxidation shaft in the vault of a reverberatory furnace has been suggested in publication Trans. AIME, 106, 1933, 104-110 (F. Laist, J. P. Cooper). The grounds for using oxygen-enriched air in the smelting of concentrates of copper and nickel have been given in publication Eng. and Min. Journ., 137, 1936, 499-502, 562-567 (T. E. Norman). Since the establishment of these basic principles of suspension techniques, and as a result of the general rise in the standards of technology, a large number of, for example, vertical suspension smelting processes have been created, and considerable improvements have also been made in these as knowledge of the art has increased.
Applications of the vertical suspension process according to both the cocurrent and the countercurrent principle include the processes acording to U.S. Pat. Nos. 2,040,682 and 2,209,331.
In general, the prior-known processes involve development of the suspension processes in order to produce higher-grade products than previously through improvements in both the process and the apparatus. The quality of the concentrates used in the suspension processes has constantly declined, while a higher degree of refining of the smelting products has been required.
Countercurrent processes are also well known in prior art. The oldest of these is probably that according to U.S. Pat. No. 1,888,164. In this process a sulfide concentrate, air and, when necessary, a fossil fuel are injected into a ceramic-lined furnace through the roof. Both the sulfur and the iron present in the concentrate are oxidized, and the smelting products thereby obtained are a molten metal phase and a molten slag phase at the base of the furnace. The reaction gases are removed through a channel in the wall of the upper section of the furnace; a heat exchanger has been provided in association with this channel for preheating the oxidation gases. The process specification does not include an example, and therefore it is not possible to evaluate the process and the operation of the smelting apparatus. It is obvious, however, that the quantity of metal obtained by the process is quite low.
In the countercurrent processes according to U.S. Pat. Nos. 2,040,682 and 2,209,331, elemental sulfur is recovered from pyrites, but the process can also be used for the treatment of sulfides of valuable metals and sulfide mattes. In the former process, which is a pure countercurrent process, a fossil fuel is used for the reduction of the sulfur dioxide gases. The completely roasted product is withdrawn from the lower end of the reactor by means of a screw (powder). In the latter process (Haglund), which includes both cocurrent and countercurrent operations, oxygen or oxygen-enriched air is used for the roasting of sulfides. The elemental sulfur is recovered from the obtained gas phase and the remaining sulfur dioxide is returned cocurrently or countercurrently to the system, and it reacts with molten sulfide, thereby releasing elemental sulfur.
The process according to Finnish Pat. No. 48202 (U.S. Pat. No. 3,900,310) already includes as an extreme case a flash-smelting furnace structure without a rising shaft; very finely-divided concentrates can be treated in it while maintaining the amount of flue dust at a moderate level. The process is thus a countercurrent process even though the concentrate and the oxidants are fed in parallel.
Of the process discussed, none is symmetrical in regard to suspension flows. In the processes which have a molten metal or sulfide phase as the product phase the suspension apparatus is in the same furnace zone as the melt.
In these prior-known processes, the suspension is a free suspension which is not rotating. Suspension flows which rotate vigorously are also used in vertical suspension processes, and naturally in all cyclone processes. There are methods of this type based on either the cocurrent principle or the countercurrent principle.
One example of the rotary-flow suspension processes carried out in a vertical shaft is Lange's suspension evaporation system (German Pat. No. 1,052,692). One countercurrent process which already very closely approaches cyclone processes is that according to British Pat. No. 1,001,310, which is used for the reduction and evaporation of zinc contents of slags and at lead smelting plants.
Water cooling devices are always used in cyclone apparatus because of the problems of lining due to the high temperature of the suspension. Since the oxidation product of the cyclone is driven to the walls of the cyclone by means of rotary flows, radiation heat exchangers are not suitable for them. Publications Tsvetnye Metally, 9, 1964, 30-39 (I.M. Rafalovich, V.L. Russo), Neue Hutte, 10, 1965, 210-216 (I. A. Onajew), Freiberger Forschungshefte, B 150, 1969, 41-65 (von Sch. Tschokin) and Erzmetall, 28, 1975, 313-322 (G. Melcher, E. Muller, H. Weigel) include good descriptions of both cocurrent and countercurrent cyclones in both horizontal and vertical apparatus. In the vertical cyclone suspension processes according to U.S. Pat. No. 3,555,164 and German Pat. No. 2,038,227, the cyclone gases and the product phases are separated from each other. Furthermore, in the processes the metal or matte space has been separated from the slag space by means of a partition wall, whereby the volatile products of the light-arc slag purification cannot come into contact with the sulfur dioxide gases. In the latter process, which is identical with the former, a complete roasting and smelting of the sulfide concentrate is carried out, in which case no sulfur is present in the valuable-metal bearing oxide melt to be reduced by electrical means.
One object of the present invention is to provide a process by which sulfide concentrates can be treated at high feed capacities in such a manner that the products obtained are sulfide mattes which are medium-grade in their valuable metal content, slags poor in valuable metals, and a gas phase which is sulfur dioxide of maximum purity directly suitable for liquefaction.
Using conventional suspension processes and technology it is difficult to meet all of these requirements simultaneously. High-capacity suspension smelting by means of oxygen results in very high heats of reaction difficult to extract from the reaction space. On the other hand, the systems corresponding to the partial pressures of sulfur dioxide have high oxygen potentials, and therefore the respective valuable-metal contents in the slags are uneconomically high. In applications of the cocurrent systems the transfer of heat by radiation from the suspension is low in spite of the high particle density, and during the process the autogenic lining derived from the shaft oxidation product accumulating on the walls of the furnace apparatus also effectively inhibits the operation of the cooling devices.